1. Field of the Invention
The present invention concerns a method and device for indexing and modifying the compression parameters of digital images.
2. Related Art
A search for images makes it possible to find, amongst an image database, those which correspond most to a criterion without having to analyse the content of each image in the database.
More precisely, the purpose of an indexing system is to associate, with each image in the base, information characteristic of the content of the image, referred to as the “index of the image”. All this information constituting what is referred to as the “index of the database”. Typically, the content of an image is characterised by its colour distribution (histogram) and its texture.
A user interrogates an image database through a request containing information characteristic of the type of image sought. Conventionally, this request consists of an example image. The user chooses an image, in the database or external to it, whose content is similar to the type of image sought.
The system next associates with this image an item of information characteristic of its content. The index thus associated with this example image is then compared with the index of the database according to a search strategy.
It is possible for example to cite the QBIC (“Query By Image Content”) system of IBM, which consists of extracting, from a non-compressed image, a description of its content.
This description consisting of characterising the content of the image by the distribution of the colours and textures of this image. For further explanations on this system, reference can be made to U.S. Pat. No. 5,579,471 of the company IBM entitled “Image query system and method”. 
However, the majority of digital images, for obvious reasons of storage problems, are stored in compressed form.
Thus, during the indexing of these images, each image must be decompressed before being able to extract the index associated with each of these images.
This decompression phase can be eliminated if the index is extracted directly from the compressed image. In this case, the time necessary for effecting the indexing phase is reduced.
There are few indexing systems which extract indexes directly from the compressed images.
It is possible notably to cite the studies carried out in order to describe the content of an image, from a discrete cosine transform (“DCT”), or from a decomposition into sub-bands.
This is for example described in “Exploiting the JPEG compression scheme for image retrieval”, IEEE Trans Patt Anal Mach Intell, 18(8), August 1996, in the case of a DCT, and by “Multiresolution video indexing for subband video databases”, Proc of SPIE Storage and Retrieval for Image and Video Databases, in the case of a decomposition into sub-bands.
It is important to note that, in these compressed image indexing systems, the compression and indexing phases are performed separately.
This is because a user defines the coding parameters according to criteria such as the minimisation of the memory location, necessary for storing the image, or so as to obtain a bit stream organised by quality levels, thus facilitating the display of the image on a distant screen.
The description of an image from a bit stream may, in some cases, not be faithful to the content of the compressed image.
Take the example where an image is compressed according to the coder into sub-bands described by FIG. 5a (described subsequently), a frequency decomposition of which is given by FIG. 6c (described subsequently). If the indexing method consists of characterising the content of the compressed image by means of a histogram calculated on the low-frequency sub-band referenced LL3 in FIG. 6c, it is necessary for the histogram to represent the visual content of the image faithfully.
The resolution RES1 of the low-frequency sub-band is fixed by the user who has compressed the image, without being preoccupied whether this compressed image would be indexed or not.
Thus, according to the resolution RES and/or the number of decomposition levels, it may happen that the resolution RES of the sub-band may be around a few pixels, which proves inadequate for obtaining a histogram representing the content of the image.